1. Field of the Invention
The present invention relates to a wavelength conversion apparatus and method in a Time Division Multiplexing Passive Optical Network (TDM-PON) system based on a Wavelength Division Multiplexing (WDM) system, and more particularly, to a wavelength conversion apparatus in a hybrid-PON system in which a TDM-PON system is mixed with a WDM system and a method of constituting a remote node (RN) using the same.
This work was supported by the IT R&D program of MIC. [2005-S-401-02, Optical Subscriber and Access Network Technology]
2. Description of the Related Art
Transmission speeds increase every year with an increase of demands for multimedia data and Internet according to demands of subscribers in homes and offices.
An X digital subscriber line (xDSL), a cable modem, etc. have been used to cope with the demands for an increase in transmission speeds. However, a bandwidth has almost reached the limits. Thus, Fiber to the x (FTTx), which performs connections to subscriber areas through optical cable, is regarded as a solution.
A PON system performs connections through optical cable to reduce the number of fibers and transceivers using an FTTx Star method and thus is regarded as being the most low-priced. Thus, several countries in the world have developed and standardized PON systems.
Representative examples of PON include a TDM-PON and a WDM-PON. The TDM-PON is representative of a Broadband PON (BPON), an Ethernet PON (EPON), and a Gigabit PON (GPON), and the WDM-PON is used to transmit data to subscribers in units of wavelengths.
However, TDM-PON and WDM-PON systems are intended for a small number of subscribers and thus are not suitable for a large scale subscriber network to which a large number of users subscribe.
The TDM-PON system connects optical cable using the Star method in order to provide super-high speed transmission services to between 16 to 32 subscribers.
The TDM-PON system allows several subscribers to share a bandwidth in the time domain. Initial installation costs for the TDM-PON system are low. However, a transmission speed is lowered with an increase in the number of subscribers, and the number of fibers must be increased in proportion to the number of subscribers in order to accommodate a large number of subscribers.
Subscribers connected to a PON share a fiber using a time multiplexing method. Thus, a transmission speed of a single subscriber is lowered with increases in the number of subscribers and a bandwidth used by each of the subscribers.
Optical cable of tens to hundreds of cores must be installed to provide services to apartments.
The WDM-PON system can solve the limitation in the bandwidth of the TDM-PON system. However, in the WDM-PON system, a transmitter must have WDM laser diodes (LDs) which radiate laser beams having different wavelengths in each optical network unit (ONU).
The WDM LDs of the ONUs have big disadvantageous in terms of price increases and management of the ONUs.
If the wavelengths of the laser beams radiated from the WDM LDs move, performance of the neighbor channel may be deteriorated, and service companies have difficulty managing stocks of the ONUs.
The WDM-PON allocates a wavelength to each of the ONUs in order to extend a bandwidth. However, each of the ONUs must have its own wavelength and thus costs increase and each of the ONUs is difficult to manage. There are not enough broadband contents to use all portions of a wavelength. Thus, the WDM-PON system has lower use efficiency of a bandwidth than the TDM-PON system.
An Injection Locking method has been suggested and developed by which general multi-mode LDs are used to radiate laser beams onto ONUs and allow incident laser beams having reference wavelengths to be radiated onto the ONUs from an external source. However, it is very costly to provide the reference wavelengths, and it is difficult to increase transmission speeds of the ONUs so as to adjust wavelengths.
Granularity is another problem of the WDM-PON system.
The TDM-PON system provides smaller units of bandwidths to subscribers, while the WDM-PON provides a single wavelength to each ONU. Thus, the WDM-PON requires an additional apparatus to divide the single wavelength into small bandwidths so that a large number of subscribers can use the small bandwidths.
The present invention provides a hybrid-PON system which can compensate for the disadvantages of the WDM-PON and TDM-PON systems.
The hybrid-PON system accommodates advantages of the TDM-PON and WDM-PON systems. Each ONU of the hybrid-PON system performs transmission and reception through a TDM-PON protocol. However, in the TDM-PON system, each group has its own wavelength and each ONU performs transmitting with each Optical Line Terminals (OLT) using a fiber so as to increase use efficiency of a bandwidth and reduce transmission costs.
The hybrid-PON system of the present invention (i.e., a TDM-PON system based on a WDM-PON system) can economically provide PON services in an area in which hundreds or more of subscribers are gathered, such as a large shopping center, an apartment, or the like.